1. Field of the Invention
The present invention relates to a thin power supply unit which is formed by a plurality of flat power supply elements such as electric double layer capacitors or batteries, for example, each comprising a case having a flat shape as a whole. More particularly, the present invention relates to an improvement in the arrangement of a plurality of flat power supply elements.
2. Description of the Background Art
With recent development of the semiconductor technique, not only industrial but consumer apparatuses generally carry CPUs. In such apparatuses, programs and data are stored in a RAM, which must be regularly supplied with a voltage of at least about 2 V for the purpose of memory protection. Such a voltage for memory protection is supplied from a power supply, which is suitably formed by a lithium battery or an electric double layer capacitor.
In general, a lithium battery contains a nonaqueous electrolytic solution which is prepared by dissolving an electrolyte of lithium perchlorate or the like in an organic solvent such as propylene carbonate or gamma-butyrolactone. Such an electrolytic solution is preferably applied also to an electrolytic double layer capacitor, since it is thereby possible to set the withstand voltage at a high value.
However, when the water content is increased in a system employing such a nonaqueous electrolytic solution, the internal resistance is increased and the withstand voltage is reduced thereby deteriorating the performance. Therefore, it is necessary to maintain the system in a highly dehydrated state.
With recent reduction in size, thickness and weight of electronic apparatuses, reduction is increasingly required also in the size and thickness (height) of a backup power supply which is carried on such electronic apparatuses. In particular as to the thickness, the power supply must be smaller than the armoring height of a general semiconductor device.
FIG. 9 shows a prior art power supply element which satifies the aforementioned requirement for reduction in height. Such a prior art power supply element has the structure of the so-called "paper lithium battery" or a structure corresponding to it. Referring to FIG. 9, a flat power supply element 1 comprises a flat case 2, which is provided with first and second case halves 3 and 4 formed by metal plates or foils of for example, stainless steel or the like. Peripheral edges of the first and second case halves 3 and 4 are bonded to each other by a sealing material 5 of an organic substance such as for example, a thermobonding film, whereby the case 2 has a closed structure. The case 2 is provided therein with first and second functional materials 6 and 7 which hold a separator 8, containing an electrolytic solution, therebetween. The first and second functional materials 6 and 7 are positive and negative electrode active materials when the power supply element 1 is a battery; while the first and second functional materials 6 and 7 are first and second polarizable electrodes when the power supply element 1 is an electric double layer capacitor. In the prior art power supply element 1, the first and second case halves 3 and 4 are electrically insulated from each other by the sealing material 5, while the first case half 3 is electrically in contact with the first functional material 6 and the second case half 4 is electrically in contact with the second functional material 7. Thus, the first and second case halves 3 and 4 serve as first and second element terminals, respectively.
In the prior art power supply element 1, it is possible to ignore the permeation of moisture inside the first and second case halves 3 and 4 themselves. However, the sealing material 5, which is prepared from an organic substance as hereinabove described, tends to allow the permeation or penetration of moisture and the vaporization of the electrolytic solution which is contained in the element, thereby leading to an inferior sealing of the prior art power supply element 1. Thus, the power supply element 1 of the prior art has only a short life, and the potential for using the element 1 is extremely limited.
In order to improve the sealing of the power supply element 1, Japanese Patent Application No. 62-296879 (1987) (Japanese Patent Laid Open Application No. 1-140553 (1989)), filed by the assignee of the present invention, proposes a flat power supply element of a closed structure with high sealability.
FIG. 10 is a sectional view showing a flat power supply element 9 according to an embodiment of the aforementioned Japanese prior art reference. Referring to FIG. 10, a case 10 of the flat power supply element 9 comprises first and second case halves 11 and 12 of for example stainless foil. The case halves 11, 12 have peripheral edge portions 13 which are welded to each other to seal the case 10. The first case half 11 is provided with an opening 14, and an electrode plate 15 of stainless foil, for example, is arranged to face this opening 14. The electrode plate 15 and the first case half 11 are insulated from each other and sealed by an insulating layer 16 which is made, for example, of resin. The case 10 which is closed in the aforementioned manner is provided therein with first and second functional materials 18 and 19, which hold a separator 17 therebetween.
According to the aforementioned structure shown in FIG. 10, the insulating layer 16 can be sufficiently increased in width substantially with no increase in the outside dimension of the power supply element 9, whereby it is possible to further effectively prevent permeation and penetration of moisture as well as vaporization of an electrolytic solution. According to the prior art flat power supply element 9, therefore, it is possible to attain remarkable improvement in life and reliability of the power supply element 9.
However, the prior art structure shown in FIG. 10 still has a problem to be solved.
A battery or an electric double layer capacitor is primarily used as a main power supply, a power supply for memory protection or an auxiliary power supply for battery exchange. The rated voltage and capacitance of the battery or double layer capacitor are varied with the application. In particular, the rated voltage is usually set at 5 to 6 V since the supply voltage for a generally employed semiconductor element is mainly 5 V, while an auxiliary power supply for battery exchange requires a higher rated voltage of about 7 V in response to the maximum voltage of the battery. The aforementioned prior art power supply elements 1 or 9 however, can attain only a low rated voltage of about 1.2 to 3.6 V for a battery, or a rated voltage of about 0.9 to 2.8 V for an electric double layer capacitor. Thus, it is necessary to connect a plurality of such power supply elements 1 or 9 in series to each other, in response to the desired rated voltage.
However, it is extremely difficult to connect a plurality of flat power supply elements 9 shown in FIG. 10, since the electrode plate 15 which serves as an element terminal in each element 9 is exposed only in the opening 14 which is provided in the case 10.